How to upgrade Kubernetes cluster with kubeadm

Testbed scheme:

The overall steps to upgrade Kubernetes cluster are:

Upgrade all nodes from v1.23.0 --> v1.24.1 To see the latest kubeadm releases go to https://kubernetes.io/releases/
Upgrade Kubernetes control plane
Upgrade worker nodes

To upgrade the control node first we need to drain the node:
```
kubectl drain [node_name] --ignore-daemonsets
```

Upgrade the kubeadm (v1.23.0 --> v1.24.1):

sudo apt-get update && \
sudo apt-get install -y --allow-change-held-packages kubeadm=1.24.1-00

Check the kubeadm version:
```
kubeadm version
```
To update the Kubernetes control plane internal components use:
```
sudo kubeadm upgrade plan v1.24.1
```
You should see the following message:
Now apply the upgrade:
```
sudo kubeadm upgrade apply v1.24.1
```
You should see the following message:

Upgrade kubelet and kubectl packages:

sudo apt-get update && \
sudo apt-get install -y --allow-change-held-packages kubelet=1.24.1-00 kubectl=1.24.1-00

Reload the daemon:
```
sudo systemctl daemon-reload
```
Restart the kubelet:
```
sudo systemctl restart kubelet
```
After an upgrade made you can now uncordon the node and back the workload to the node again:
```
kubectl uncordon [control_node_name]
```
Check the upgrade with the following command:
```
kubectl get nodes
```
You now should see that control node is upgraded to version 1.24.1:

Now drain the worker node (control node):
```
kubectl drain [worker_node_name] --ignore-daemonsets --force
```
ATTENTION: --force flag is dangerous as it will evict all standalone pods not controlled by Kubernetes controller objects

Now log into your drained worker node and upgrade the kubeadm:

sudo apt-get update && \
sudo apt-get install -y --allow-change-held-packages kubeadm=1.24.1-00

Upgrade the kubelet and kubectl packages:

sudo apt-get update && \
sudo apt-get install -y --allow-change-held-packages kubelet=1.24.1-00 kubectl=1.24.1-00

Reload the daemon:
```
sudo systemctl daemon-reload
```
Restart the kubelet:
```
sudo systemctl restart kubelet
```
Now go back to the control node and uncordon the worker node:
```
kubectl uncordon [worker_node_name]
```
Check if worker node is upgraded:
```
kubectl get nodes
```
You should see the following:
Repeat the same process for the rest of worker nodes

Node draining is the mechanism that allows users to gracefully move all containers from one node to the other ones.
A DaemonSet ensures that all eligible nodes run a copy of a Pod. Normally, the node that a Pod runs on is selected by the Kubernetes scheduler. However, DaemonSet pods are created and scheduled by the DaemonSet controller instead.
--allow-change-held-packages - overwrites the hold status from kubeadm package. Hold status prevents any upgrades of the package
systemctl is a Linux command-line utility used to control and manage systemd and services. You can think of Systemctl as a control interface for Systemd init service, allowing you to communicate with systemd and perform operations. Systemctl is a successor of Init.

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